TOXODONTS. 
565 
The Toxodoxts. 
Suborder Toxodontia. 
The toxodonts may be defined as a group of more or less aberrant Ungulates 
with tall-crowned and curved cheek-teeth, some or all of which grow from 
persistent pulps, either permanently or during life; while at least one pair of 
incisors in each jaw are rootless, and the third upper incisor, when present, is 
placed in the line of the cheek-teeth. The vertebne of the neck are short, with 
flattened articular faces to the bodies, and the vertebral artery piercing the 
transverse process in the ordinary manner. The wrist (when known) is of the 
alternating type, while the ankle-joint is formed on the linear plan. The astragalus 
is slightly grooved on its superior face, and interiorly is like that of the Odd-toed 
group, having no head for the navicular; but the calcaneum, which is truncated 
interiorly, has a large articular surface for the fibula, as in the Even-toed section. 
The number of toes varies from five to three; the middle one being larger than 
either of the others, and symmetrical in itself. The femur may or may not have a 
third trochanter. The number of trunk-vertebrae in the typical genus is twenty, 
or intermediate between that of the Even and Odd-toed groups. In form the 
cheek-teeth of the less specialised forms approximate to the Odd-toed plan of 
structure; and in all the genera the enamel is most developed on, or even confined 
to, the outer sides of the cheek-teeth, although there may be vertical bands on some 
of the other surfaces. More specialised in the structure of the feet and teeth than 
the last group, phylogenetically the toxodonts may apparently be regarded as 
related to the Odd-toed Ungulates, but as retaining certain features now common 
to the Even-toed group, which have probably been inherited from common 
ancestors. 
While, as aforesaid, the more generalised members of the suborder approximate 
in the structure of their teeth and feet to the Odd-toed group, the specialised forms 
assume a more or less Rodent-like type of dentition and limb-structure, which 
must probably be regarded as an instance of parallelism. It may be added that, 
from the retention of clavicles, these Rodent-like types must be derived from some 
form less specialised than toxodon, in which those bones have disappeared. 
The toxodon was of the size of a large rhinoceros, and characterised by the 
long and curved crowns of its molar teeth, which continued to grow throughout 
life. There were only two pairs of incisor teeth and no tusks in the upper jaw, 
although in the lower jaw the full number of these teeth were developed. The feet 
were furnished with three toes. 
This genus occurs in the superficial deposits of Argentina, but is replaced in 
the Miocene Tertiaries of Patagonia by certain allied forms known as nesodons, 
which may be briefly defined as including toxodonts of medium or small size, in 
which the limbs, and probably also the neck, were relatively longer and more 
slender than in the typical genus; while all the teeth, with the exception of the 
second upper and third lower incisors developed roots in the adult state, and the 
upper molars were of a type approaching that of the Odd-toed group, with a 
distinct posterior valley, and the middle column forming a distinct lobe projecting 
